init: Add "export_rc" command to export env from a specified file

New export_rc command to import environment variables from
a specified file at the time the command is executed.

Differences between "import" and "export_rc":
1) export_rc can only import environment vars
2) export_rc is performed when the command
   is executed rather than at the time the
   command is parsed (i.e. "import")

Example:
in /init.rc:
    on post-fs
        export_rc /system/etc/init.xyz.rc

in /system/etc/init.xyz.rc:
    export LD_PRELOAD /system/lib/libxyz.so

Change-Id: If616dea5b508c27e9f7a95b5e9db696ecea2fdee